This invention relates to a variable delivery fluid pump and, more particularly to a common rail fuel system that utilizes the pump to supply actuation fluid to a common fluid accumulator or rail.
In a common rail fuel injection system, high pressure actuation fluid is used to power electronic unit injectors, and the actuation fluid is supplied to the injectors from a high pressure fluid accumulator, which is referred to as a rail. To permit variation of the fluid pressure supplied to unit injectors from the rail, it is desirable to vary the delivery of fluid to the rail from one or more actuation fluid pumps. Known common rail systems typically rely on either a single fluid pump that supplies fluid to the rail or a plurality of smaller displacement pumps that each supplies fluid to the rail. The volume and rate of fluid delivery to the rail has been varied in the past by providing a rail pressure control valve that spills a portion of the delivery from a fixed delivery pump to maintain the desired rail pressure.
Variable delivery pumps are well known in the art and are typically more efficient for common rail fuel systems than a fixed delivery actuation fluid pump, since only the volume of fluid need to attain the desired rail pressure must be pumped. For example, variable delivery has been achieved from an axial piston pump, e.g. a pump wherein one or more pistons are reciprocated by rotation of an angled swash plate, by varying the angle of the swash plate and thus varying the displacement of the pump. In such a pump, the swash plate is referred to as a xe2x80x9cwobble platexe2x80x9d. Variable delivery has also been achieved in fixed displacement, axial piston pumps by a technique known as sleeve metering, in which each piston is provided with a vent port that is selectively closed by a sleeve during part of the piston stroke to vary the effective pumping portion of the piston stroke.
While known variable delivery pump designs are suitable for many purposes, known designs are not always well suited for use with modern hydraulically actuated fuel systems, which require fluid delivery to the rail to be varied with high precision and with rapid response times measured in microseconds. In addition, known variable delivery pumps designs are typically complex, may be costly, and are subject to mechanical failure.
In one specific example, European patent application 307,947 of NIPPONDENSO CO.,LTD. shows a variable discharge fixed displacement high pressure pump that utilizes an electronically actuated pressure latching valve in order to control output from the pump. When this pump begins its pumping stroke, fluid from the pumping chamber can either be displaced back to the inlet or out of the outlet. At any time during the pumping stroke, an electronically actuated spill valve can be actuated to close the spill passage between the pump chamber and the inlet to the pump. When this occurs, pressure in the pumping chamber quickly rises, and the spill valve includes a closing hydraulic surface that holds it closed due to the high pressure in the pumping chamber. When the valve is closed, the fluid exits the pump through the outlet at high pressure. Once the valve is closed and sufficient pressure is present to hold the valve in its closed position, the solenoid can be deenergized and the valve will remain in its closed position. While the concept of using a pressure latching valve can be beneficial from the standpoint of conserving electrical energy, the NIPPONDENSO pump suffers from a number of drawbacks. First, because the flow area past the valve must be relatively large in order to accommodate the fluid displacement occurring during the pumping stroke, the spill valve must necessarily have a relatively large and heavy valve member, and a relatively long travel distance in order to have a sufficiently large flow area when the valve is in its open position. The result of this is to require a relatively large and strong solenoid, and acceptance of relatively long response times that are required to move the valve from its open position to its closed position. Because such a structure inherently causes conflicts between the control requirements and the flow requirements, the performance capabilities of the same must necessarily be compromised.
This invention is directed to overcoming one or more of the problems described above.
In one aspect of this invention, a variable delivery pump comprises a pump housing defining a pump chamber, a pump inlet and a pump outlet. At least one plunger is positioned to reciprocate in the pump housing. A by-pass valve including an electrically operated actuator and a valve block is attached to the pump housing and defines a valve inlet fluidly connected to the pump chamber. The by-pass valve further includes a primary closure member movably positioned in the valve block and a secondary closure member movably positioned in the valve block and operably coupled to the electrically operated actuator.
In another aspect of the invention, a fuel injection system comprises a common rail, a plurality of fuel injectors fluidly connected to the common rail, a source of fluid, and at least one variable delivery pump with a pump outlet fluidly connected to the common rail and a pump inlet fluidly connected to the source of fluid. The variable delivery pump comprises a pump in accordance with the preceding aspect of this invention.
In still another aspect of the invention, a method of controlling output from a variable delivery pump comprises the steps of (a) providing a variable delivery pump including at least one plunger positioned to reciprocate in a pump housing, a by-pass valve including an electrically operated actuator and a valve block attached to the pump housing and defining a valve inlet fluidly connected to a pump chamber, and further including a primary closure member movably positioned in the valve block, and a secondary closure member movably positioned in the valve block and operably coupled to the electrically operated actuator; (b) determining a desired effective pumping stroke for the variable delivery pump; and (c) closing the by-pass valve at a timing corresponding to the desired effective pumping stroke at least in part by moving the secondary closure member to a closed position and then applying a hydraulic force to move the primary closure member to a closed position.